This invention relates to apparatus and/or systems for controlling the threshold setting of field effect memory devices such as MNOS transistors and it is an object of the invention to provide improved apparatus and systems of this nature.
Field effect semiconductor devices known as MNOS transistors are very desirable for use as memory devices because when once the gate threshold voltage thereof has been set it retains that setting for a long time thereafter even though the setting voltage and all bias voltages are removed. Moreover, the threshold value of MNOS transistors can be changed from a strong enhancement mode where the threshold is at -5 to -10 volts to a strong depletion mode where the threshold is +5 to +10 volts, for example. This property of MNOS devices increases their potential of use very substantially because the setting can be changed to meet requirements of any given situation.
Ordinarily, setting the threshold of MNOS transistors is accomplished by applying relatively large positive or negative voltage pulses to the gate electrode. The principal problem is that of obtaining the desired threshold voltage without a lot of cut and try. The application of large pulses of a fixed amplitude will move the threshold from one value to another but it is necessary to stop the process when the desired condition of "on" or "off" is reached. In the past, this has been a hit or miss, or trial and error, process in which several pulses are applied to the threshold of the device following which a measurement is made to determine the threshold value. And, if not correct, additional pulses are applied and further measurements made. The process is continued until a desired value is reached. This, of course, is time consuming and tedious and it is therefore an object of the invention to provide an improved system for setting the threshold value of a field effect MNOS device which overcomes the deficiencies of the prior art.
It is a further object of the invention to provide an improved system and apparatus for controlling the threshold setting of a field effect MNOS device which automatically terminates the setting process when the desired threshold value has been reached. This makes it possible to easily obtain the same threshold setting even when NMOS devices with different characteristics are being used.
It is a further object of the invention to provide improved apparatus and systems of the nature indicated which are simple in form, easy to use, and accurate in operation.
It is a further object of the invention to provide an improved apparatus and system of the nature indicated which utilizes a feedback circuit to control the termination of setting threshold of an MNOS to the value desired.